1. Field of the Invention
The present invention relates to a system for read/write head servo control in data storage devices, and more particularly to a secondary actuator system.
2. Description of Related Art
Numerous technologies have been developed to improve the accuracy of read/write head positioning systems of information storage devices, such as disk drives, as part of the ongoing process of increasing recording density. At the high recording densities that have been achieved, the sway mode vibration of arms supporting the heads that are generated in seek operations are one of the limiting factors on the servo system accuracy. High-bandwidth servo actuators are crucial to achieving large track densities in rotating disk magnetic storage devices. Approaches to achieving higher servo actuator bandwidth include the use of high specific stiffness materials such as AIBC for making the arms, secondary actuators, and active/passive damping techniques.
The use of dual-stage actuation systems having a primary actuator, e.g., a Voice Coil Motor (VCM), for executing large movements and a secondary actuator, e.g., a piezoelectric element (PZT), for fine-tuning and tracking is well-known in the art. Small PZT milliactuators have higher vibrational modes than the VCM due to scaling. Descriptions of such systems are found in scientific and patent literature. Some representative references include Ahi-Min Y., et al., xe2x80x9cController Design Criteria for the Dual-Stage Disk Actuator Systemxe2x80x9d, Proc. SPIExe2x80x94International Society for Optical Engineering (USA), Vol. 2101, No. 1,1993, pp. 305-8 and Guo, W., et al., xe2x80x9cDual Stage Actuators for High Density Rotating Memory Devicesxe2x80x9d, IEEE Trans. Magn. (USA), Vol. 34, No. 2, pt. Mar. 1, 1998, pp. 450-5.
A PZT transducer is suitable for actuation purposes and is generally controlled by applying a suitable control voltage. In response, the PZT changes its physical dimensions causing a deflection or displacement of the PZT element and displacement of any other mechanical element to which the PZT is rigidly or semi rigidly mounted. Additionally, certain PZTs such as piezoelectric ceramics are inexpensive and combine simultaneously actuation and sensing capabilities. In particular, such xe2x80x9csmartxe2x80x9d PZTs or self-sensing PZTs react to a change in mechanical load by changing their electrical properties, including stored charge, which can be sensed by proper electrical circuitry. They can thus sense the deformation of the PZT or deflection of the arm while at the same time performing actuator functions. Self-sensing PZTs are described in prior art references such as Janocha H., et al. xe2x80x9cPrinciple of Smart Piezoactuatorsxe2x80x9d, Actuator 96, 5th International Conference on New Actuatorsxe2x80x9d, Jun. 26-28, 1996, Bremen, Germany, and Near C. D., xe2x80x9cPiezoelectric Actuator Technologyxe2x80x9d, SPIE International society for Optical Engineering, Vol. 2717, April 1998, pp. 246-58. Additional prior art references teach the use of PZTs as devices for active damping of vibrating structures or arms. Publications describing these applications of PZTs include: xe2x80x9cPiezoelectric Modal Sensor/Actuator Devices for DASD Active Damping Vibration Controlxe2x80x9d, W. W. Chiang, et al., IBM Technical Disclosure Bulletin, Vol. 34, No. 4B, September, 1991, pp. 53-4 and xe2x80x9cActive Vibration Damping of Scanning Tunneling Microscopexe2x80x9d, J. R. Kirtley, et al., IBM Technical Disclosure Bulletin, Vol. 31, No. Jul. 2, 1988, pp. 426-9.
A PZT element used as a milliactuator for deflection control as well as sensing has to be compensated for external influences of temperature, moisture and any electrical fields. Lack of proper compensation of the milliactuator will result in inferior sensing performance or even make it impossible to separate the driving signal causing the deformation from the self-sensing signal. Only when the self-sensed signal is accurate can it be used for compensating the arm assembly, e.g., compensating for vibrational modes. Dual-stage actuation systems are further described in xe2x80x9cDual Stage Actuator Servo Control for High Density Disk Drivesxe2x80x9d, L. Guo, et al., Proceedings of the 1999 IEEE/ASME International Conference on Advanced Intelligent Mechatronics, Sep. 19-23, 1999.
Secondary actuator systems that use the same element as both an actuator and a sensor generally require complicated electronics to multiplex the actuation and sensor signals. Self-sensing milliactuator systems are described in co-pending U.S. patent application Ser. No. 09/139,541 xe2x80x9cPiezoelectric Actuator for Control and Displacement Sensing,xe2x80x9d Fu-Ying Huang et al., as well U.S. patent application Ser. No. 09/272,941 xe2x80x9cDisk Drive with Mode Canceling Actuator,xe2x80x9d Imaino et al. both of these patent applications are assigned to the assignee of the present invention, and are hereby incorporated by reference herein.
A secondary actuator system is also described in U.S. Pat. No. 5,459,383, xe2x80x9cRobust Active Damping Control System,xe2x80x9d Sidman et al. This patent describes a secondary actuator system where the motion sensor is mounted on or very near the actuator motor. Placing the motion sensor near the actuator motor limits the vibration modes that the sensor will detect and therefore the vibration modes that can be compensated by the secondary actuator system.
In view of the foregoing, it is apparent that there is a need for an improved secondary actuator system to overcome the above and other problems and limitations of conventional systems.
It is an object of the present invention to provide a highly accurate vibrational mode canceling secondary actuator system that compensates for in-plane vibration modes of rotary actuator arms, and overcomes the problems of the prior art.
In summary, the present invention is a secondary servo actuator system for use in storage devices, such as disk drives. In one embodiment, two PZT transducers are mounted on each suspension load beam in an arm assembly. When a seek operation is initiated the acceleration of the arms generates vibration modes. The PZT transducers on an arm adjacent to the target arm sense the vibration motion and provide output signals to a controller. The controller filters the signal and generates a control signal for the PZTs mounted on the target arm. The deformation of the PZTs in turn bends in plane (or rotates) the arm load beam and thereby adjusts the position of the target head to compensate for arm motion caused by vibration modes. Mounting the PZT transducers on the load beam or between the load beam and arm, provides enhanced sensing capabilities compared to sensors mounted farther away from the read/write head. Using one milliactuator to sense motion and a second milliactuator to compensate for the motion avoids the limitations and complicated electronics associated with distinguishing sensed motion from the driving signal deforming the PZT.
The present invention secondary actuator system can be used during servo control about a desired deceleration trajectory of a target head to suppress lower order arm sway frequencies. The milliactuator of the target head can also be used to sense the phase and amplitude of the dominant arm sway mode to enable the deceleration control signal to be applied with a phase relation that suppresses the arm sway mode that was excited upon application of seek servo actuator control. Furthermore, milliactuators on the arms at the top and bottom of the arm assembly can be used to determine the degree of actuator assembly tilt during a seek operation. The degree of tilt can be used as a pass/fail criteria to control the radial stiffness axial asymmetry of the system which can be compromised due to pivot bearing assembly variance.
In another embodiment of the present invention the milliactuators are mounted on a flex circuit of an integrated lead suspension.